This is a pretty sad attempt to update a classic favorite.First off, do not expect any real class here. The sets are pretty good, and the veteran actors try their best, but you never get the sense that this is anything but bad TV.
As far as the differences between this and the classic film, none really get in the way of the 1975 effort. First, and most obvious, the setting is the then-present day instead of the turn-of-the-century period depicted in the classic 1946 film. Second, Helen is not a young servant girl, but a full grown widow, who third, sees her husband and daughter die in a fire, rather than witnessing her parents die as a little girl. Love interest Dr. Rawley is no longer just a simple country doctor, but instead a phychiatrist trying to treat Helen's disorder (I won't go into the ethics of a doctor getting romantic with a patient, but there it is). Lastly, many of the character names have been changed, though for what reason I can't say (perhaps going back to the originals in the book?).
Jacqueline Bisset is suitable as the mute Helen, and Christopher Plummer and John Phillip Law both deliver as the two brothers. Elaine Stritch is pretty good as the crabby nurse, while Mildred Dunnock is equally good as the bedridden matriarch of the Sherman clan. If there is an improvent over the original, it would probably have to be Gayle Hunnicutt as Blanche, Plummer's assistant (couldn't resist that one, but she's actually pretty good).
Director Peter Collinson does okay with set-ups and design, but fails miserably when it comes to bringing out the best in the actors. Most laughable is the pathetic and extra-hammy performance by John Ronane as love interest Dr. Rawley. At times, I thought I was watching a old kung fu movie, what with his over-expressive "acting" and "emoting". Sam Wanamaker is okay as the police detective, but was clearly directed to go over-the-top, just like Ronane. Collinson is probably best remembered for a Michael Caine film, "The Italian Job". In addition to this remake, he also did an overseas remake of "And Then There Were None".
Although released theatrically, it sure feels like a made-for-TV movie, and it may be interesting for comparison with the original film. I think you should see this version if you're a fan of the original, or mid-70's dreck.
Fans of the 70's will get a real hoot out of the dated style of phone Helen tries to use, too (I know I did).